Polystyrene nanocomposite optical plastic article and method of making same

ABSTRACT

An optical nanocomposite material has a nanoparticulate filler dispersed in a host plastic material. According to the method of making the nanocomposite material, a predetermined temperature sensitive optical vector, such as refractive index, of the plastic host material and nanoparticulate filler are directionally opposed resulting in a nanocomposite material having significantly improved stability of the refractive index with respect to temperature.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is related to U.S. application Ser. No.09/748,634, filed Dec. 22, 2000, by Border, et al., and entitled,“Polymethylmethacrylate Nanocomposite Optical Plastic Article And MethodOf Making Same,” U.S. application Ser. No. 09/748,635, filed Dec. 22,2000, by Border, et al., and entitled, “Cyclic Olefin PolymericNanocomposite Optical Plastic Article And Method Of Making Same,” U.S.application Ser. No. 09/747,705, filed Dec. 22, 2000, by Border, et al.,and entitled, “Reduced Temperature Sensitive Polymeric Optical ArticleAnd Method Of Making Same,” U.S. application Ser. No. 09/747,706, filedDec. 22, 2000, by Border, et al., and entitled, “PolycarbonateNanocomposite Optical Plastic Article And Method Of Making Same,” U.S.application Ser. No. 09/747,707, filed Dec. 22, 2000, by Border, et al.,and entitled, “Polysulfone Nanocomposite Optical Plastic Article AndMethod Of Making Same.”

FIELD OF THE INVENTION

The invention relates generally to the field of polymeric opticalarticles. More particularly, the invention concerns polymeric opticalmaterials and articles, such as plastic lenses, that maintain stableperformance characteristics over a broad temperature range.

BACKGROUND OF THE INVENTION

Plastic lenses and glass lenses often perform the same function inoptical systems, such as in cameras, microscopes, telescopes andopthalmic wear. The two main attributes that separate plastic lensesfrom glass lenses are cost and optical stability. Plastic lensestypically cost {fraction (1/100)}^(th) the price of a similar glasslens. On the other hand, the stability of the refractive index of aglass lens with respect to temperature and humidity is typically 100times better than that of a plastic lens.

The difference in cost is due largely to the difference in manufacturingprocesses that are required for the two materials and the relativetemperatures that the materials must be formed at. Plastic lenses aretypically produced at 230° C. using injection molding at cycle timesthat are 10 times faster than glass lenses that are largely produced bygrinding and polishing or compression molding at 625° C. Grinding andpolishing are labor intensive while the high temperatures that glassmust be formed at requires expensive mold materials and extensivemaintenance costs.

In contrast, the difference in optical stability between plastic andglass is due to differences in their basic material properties. Thisdifference in optical stability results in substantially more variationin focus and image quality in articles such as cameras when plasticlenses are used in place of glass. What is desired, and a remainingchallenge in the art, is a material with the optical stability of glassthat processes like a plastic. While optical plastic materials such ascyclic olefins greatly improve the refractive index stability withrespect to humidity, improving the refractive index stability withrespect to temperature has remained an opportunity. A study on thecompeting fundamental material characteristics that determine the signand the magnitude of the dn/dT of glasses is available, for instance, byLucien Prod'homme, “A new approach to the thermal change in therefractive index of glasses,” Physics and Chemistry of Glasses, Vol. 1,No. 4, Aug. The two competing effects that determine the dn/dT inglasses are the density change which produces a negative dn/dT and theelectronic polarizability which produces a positive dn/dT. The net dn/dTin a glass material depends on which effect dominates. In opticalplastics however, there is not an electronic polarizability so that allunfilled materials have negative dn/dT values. Nonetheless, the articleby Prod'homme does identify the possibility of using glass-like fillerswith positive dn/dT values to substantially alter the dn/dT of a filledplastic composite material.

Nanoparticulate fillers have been used to modify the index of refractionof optical plastics. By using a filler small enough that it is wellbelow the wavelength of visible light (400-700 nm), the filler will notscatter the light and the filled plastic can retain its transparency.WIPO Patent WO97/10527 describes the use of nanoparticles to increasethe refractive index of plastics for opthalmic applications. Inaddition, technical references that describe the addition ofnanoparticles to increase the refractive index of plastics include: C.Becker, P Mueller, H. Schmidt; “Optical and ThermomechanicalInvestigations on Thermoplastic Nanocomposites with Surface-ModifiedSilica Nanoparticles,” SPIE Proceedings Vol. 3469, pp. 88-98, July 1998;and, B. Braune, P. Mueller, H. Schmidt; “Tantalum Oxide Nanomers forOptical Applications,” SPIE Proceedings Vol 3469, pp. 124-132, July1998. While these references disclose the use of nanoparticles to modifyrefractive index of optical plastics they do not discuss the issue ofrefractive index stability with respect to temperature which requires adifferent set of characteristics in the nanoparticle.

U.S. Pat. No. 6,020,419 issued to M. Bock, et al., discloses the use ofnanoparticulate fillers in a resin based coating for improved scratchresistance. U.S. Pat. No. 5,726,247 issued to M. Michalczyk, et al.,also describes a protective coating that incorporates inorganicnanoparticles into a fluoropolymer. While scratch resistance isimportant in plastic optics, the nanoparticles that would be suitablefor scratch resistance would be very different from those with thespecific properties needed to improve refractive index stability withrespect to temperature.

U.S. Pat. No. 3,915,924 issued to J. H. Wright describes ananoparticulate filled clear material for filling voids. U.S. Pat. No.5,910,522 issued to H. Schmidt, et al., describes an adhesive foroptical elements that includes nanoscale inorganic particles to reducethermal expansion and improved structural properties at elevatedtemperatures. While the inventions described in these patents representssome progress in the art, none of them address specific opticalproperties of the modified plastic material particularly as theseproperties relate to temperature sensitivity.

WIPO Patent WO99/61383A1 a discloses a method for producing multilayeredoptical systems that uses at least one layer that containsnanoparticulate fillers to form a layer with a different refractiveindex than the substrate to create an interference filter or anantireflection layer. Obviously, this patent is addressing another formof modification of the index of refraction and such is not concernedwith the stability of the index of refraction with respect totemperature.

Skilled artisans will appreciate that a wide variety of materials areavailable in nanometer particle sizes that are well below the wavelengthof visible light. Representative materials may be acquired fromcompanies such as Nanophase Technologies Corporation and NanomaterialsResearch Corporation. By selecting nanoparticle materials based onproperties other than index of refraction, our experience indicates thatit is now possible to modify other optical properties of plastics.

While there have been several attempts to modify properties of plasticsusing nanoparticles, none of these attempts have proven successful inproducing optical plastic articles with temperature stable opticalproperties while retaining important processing characteristics.

Therefore, a need persists in the art for optical plastic articles, suchas lenses, and a method of making same that have temperature stableoptical properties.

SUMMARY OF THE INVENTION

It is, therefore, an object of the invention to provide an opticalnanocomposite material that has reduced temperature sensitivity.

Another object of the invention is to provide an optical article, suchas a plastic lens, that maintains stability over a broad range oftemperatures.

Yet another object of the invention is to provide a method ofmanufacturing an optical article having reduced temperature sensitivity.

It is a feature of the optical article of the invention that a selectnanoparticulate dispersed into a plastic host material having atemperature sensitive optical vector that is directionally opposed tothe temperature sensitive optical vector of the nanoparticulate filler.

To accomplish these and other objects, features and advantages of theinvention, there is provided, in one aspect of the invention, apolystyrene nanocomposite optical plastic article comprising: apolystyrene host material having a temperature sensitive optical vectorx₁ and nanoparticles dispersed in said polystyrene host material havinga temperature sensitive optical vector x₂, wherein said temperaturesensitive optical vector x₁ is directionally opposed to said temperaturesensitive optical vector x₂.

In another aspect of the invention, there is provided a method ofmanufacturing a polystyrene nanocomposite optical plastic article,comprising the steps of:

(a) providing a polystyrene host material having a temperature sensitiveoptical vector x₁ and nanoparticles having a temperature sensitiveoptical vector x₂, wherein said temperature sensitive optical vector x₁is directionally opposed to said temperature sensitive optical vectorx₂,

(b) dispersing said nanoparticles into said polystyrene host materialforming a polystyrene nanocomposite material; and,

(c) forming said polystyrene nanocomposite material into saidpolystyrene nanocomposite optical plastic article.

Hence, the present invention has numerous advantageous effects overexisting developments, including: (1) the resulting nanocomposite has asignificantly lower dn/dT (change in refractive index with temperature);(2) lenses made with the nanocomposite material have more stable focallength over a given temperature range; (3) low levels of dn/dT areachievable in the nanocomposite material with reduced loading of thenanoparticulate; (4) the viscosity of the nanocomposite material is notsignificantly higher than the base plastic so that conventional plasticprocessing techniques can be used; and, (5) the nanocomposite materialhas improved barrier properties so that the change of refractive indexwith respect to humidity will be reduced compared to the base plastic.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features, and advantages of the presentinvention will become more apparent when taken in conjunction with thefollowing description and drawings wherein identical reference numeralshave been used, where possible, to designate identical features that arecommon to the figures, and wherein:

FIG. 1 is a plastic lens showing a range of focal length variationproduced by a change in temperature and the resulting change inrefractive index;

FIG. 2a shows a lens made from a nanocomposite material that hasimproved stability of refractive index with respect to temperature andan associated reduced range of focal length variation produced by achange in temperature;

FIG. 2b shows a representative view of the nanocomposite material beforeforming into an optical article;

FIG. 3 is a block diagram of the process for manufacturing a plasticoptical article with improved refractive index stability;

FIG. 4 is a schematic diagram of a nanocomposite material making processbased on compounding; and,

FIG. 5 is a schematic diagram of a nanocomposite material making processbased on solvent dispersion.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1, it is well known that in a typical prior artlens 1, focal length varies significantly with changes in temperature(T). The relationship between focal length and refractive index is givenby the below equation:

f=R/(n−1);  Equation 1

wherein (f) is the focal length of the lens 1 produced as incident light3 goes through the lens 1 and is focused at focal point 5, (R) is theradius of the lens surface, and (n) is the refractive index of the lensmaterial. In the case of a camera lens (not shown), the temperaturerange of operation can easily be 50° C. when used to photograph atropical island and then later used to photograph a snowy mountain. Asan example, a lens 1 having a 10 mm radius and made, for instance, ofpolymethylmethacrylate, the index of refraction (n) at room temperatureis 1.492 and the focal length (calculated from Equation 1 above) is20.325 mm.

In a typical prior art lens 1 comprising a plastic material selectedfrom Table I, the change in refractive index (dn) over the temperaturerange of operation is 0.0055 and the change in focal point 5 shown asthe change in focal length 7 (FIG. 1) of the lens from Equation 1 is0.225 or 1%. Skilled artisans will appreciate that the image quality ofimages made with the lens 1 will not be the same over the entireoperating temperature range due to variations in focus quality.

Turning now to FIG. 2a, the reduced temperature sensitive, nanocompositeoptical article or lens 10 of the invention is illustrated. According toFIG. 2a, the nanocomposite optical article or lens 10 is composed of aplastic host material 16 and a select nanoparticulate material dispersedin the plastic host material 16. Polymeric host material 16 may beeither a thermoplastic or thermoset material. It is important to theinvention that the polymeric host material be selected based on apredetermined temperature sensitive optical vector x₁, for instancerefractive index n. Similarly, the selection of the nanoparticulatematerial dispersed in the polymeric host material 16 must be based on acorresponding predetermined temperature sensitive optical vector x₂,specifically refractive index. In this case, temperature sensitiveoptical vectors x₁, and x₂ are defined by a change in refractive index(dn) of the polymeric host material 16 and the nanoparticulate material,respectively, with respect to a change in temperature (dT). It isfurther important to our invention that x₁ is directionally opposed tox₂. By carefully selecting a nanoparticulate material having a dn/dT,i.e., a rate of change of refractive index with respect to temperature,that has a sign that is directionally opposed to the dn/dT of thepolymeric host material 16, it is possible to significantly reduce thedn/dT of the resulting nanocomposite material at relatively low loadingsof the nanoparticulate material. As a result, the viscosity of thenanocomposite material is not drastically increased and the processingcharacteristics will be similar to other optical plastics. Consequently,the resulting optical nanocomposite lens 10 has a focal length range 12(FIG. 2a) over the operating temperature range that is much less thanthat exhibited by the prior art lens 1 shown in FIG. 1. According toTables I and II, several select dn/dT values for polymeric hostmaterials (plastics) and inorganic nanoparticulate fillers that comprisethe nanocomposite material of the invention are illustrated.

TABLE 1 Approximate dn/dT for Various Optical Plastics Plastic dn/dT(10⁻⁶/° C.) Polymethylmethacrylate −105 Polystyrene −127 Polycarbonate−114 Polystyrene −110 Cyclic olefin copolymer −102 Polysulfone −100

TABLE 2 Approximate dn/dT for Various Inorganic Materials withTransmission Bands in Visible Wavelengths Material dn/dT (10⁻⁶/° C.)Barium fluoride −16 Aluminum oxide 14 ALON 12 Berryllium oxide 10 BBO−16 Diamond 10 Calcium carbonate 7 Calcium fluoride −10 Cesium bromide−85 Cesium iodide −99 Potassium bromide −42 Potassium chloride −36Potassium fluoride −23 Potassium iodide −45 Potassium titano phosphate12 Lithium borate −7 Lithium fluoride −17 Lithium iodate −80 Magnesiumaluminate 9 Magnesium oxide 19 Sodium bromide −40 Sodium chloride −35Sodium fluoride −13 Sodium iodide −50 Silicon oxide −5 Quartz 12Tellurium oxide 9 Titanium dioxide −1 Yttrium oxide 8 Zinc Sulfide 49

In addition to the polymeric host material 16 and the nanoparticulatematerial having directionally opposed dn/dT, the invention contemplatesother qualifications for the nanoparticulate material to make theuseful, novel and unobvious optical nanocomposite material of theinvention. For instance, the nanoparticulate material must betransparent in the wavelength region of interest to maintain highoptical transmission levels. Moreover, the nanoparticulate must beavailable in a particle size range that is less than 40 nm to avoidscattering light. Most preferred is a particle size range below 20 nm.Further, it must be possible to disperse the nanoparticles into the baseor host plastic such that no significant amounts of agglomerates and/orvoids larger than 40 nm occur which would scatter light. FIG. 2b shows arepresentative view 15 of the nanoparticles 14 dispersed into theplastic host material 16. The nanoparticles 14 are shown dispersedevenly throughout the host material 16. The nanoparticles 14 do not haveany larger agglomerates or voids associated with them. Furthermore, thecost of the nanoparticulate and any associated surface treatments of thenanoparticles to improve dispersibility must be low enough that thetotal cost of the optical article is significantly less than a glassarticle.

As illustrated in Tables I and II, there exists a number of inorganicmaterials that have dn/dT values with an opposite sign compared topolymeric host materials. Thus, a nanocomposite material withsignificantly improved refractive index stability with respect totemperature can be formulated by dispersing a select nanoparticulatematerial into a polymeric host material 16 that have directionallyopposed (or opposite signs) dn/dT.

According to another aspect of the invention, a method of manufacturinga reduced temperature sensitive optical article or lens 10 (as describedabove) includes the step of selecting a polymeric host material 16, suchas one described in Table 1. According to the invention, the selectedpolymeric host material 16 has a temperature sensitive optical vector x,or dn/dT, as described above. A nanoparticulate material (Table II) isselected for dispersing in the plastic host material 16. The selectnanoparticulate material, according to the invention, is required tohave a compatible corresponding temperature sensitive optical vector x₂.Moreover, it is further important to the invention that x₁ isdirectionally opposed to x₂, i.e., one of the two must be negative andthe other positive. Once the nanoparticulate material is selected, it isthen dispersed in the host material 16 using suitable dispersiontechniques, such as compounding or solvent dispersion. Once thenanoparticulate material is dispersed into the polymeric host material16, a nanocomposite material is formed. The nanocomposite material canthen be used to form an array of optical articles such as the lens 10 ofthe invention having reduced temperature sensitivity.

Referring to FIG. 3, a diagram of the method 20 for making the reduceddn/dT nanocomposite material for optical articles, such as lens 10, isdepicted. First the polymeric host plastic material 22 is selected basedon optical, structural and thermal design considerations such as %transmission, % haze, index of refraction, yield strength at atemperature, impact strength, scratch resistance, glass transitiontemperature, etc. Second, the nanoparticulate material 24 is preferablyselected based on dn/dT, transparency in the wavelength region ofinterest, particle size, cost, and availability. As disclosed in thisinvention, selecting suitable nanoparticulate materials 24 requiresselecting materials having a dn/dT that has a sign that is opposite tothe host plastic material being used and an average particle size lessthan about 40 nm. Third, the nanoparticles are preferably dispersed 26into the host material although other mixing processes could be used,such as roll milling. Dispersion 26 can be accomplished throughpreferably compounding (refer to FIG. 4) even though solvent dispersion(refer to FIG. 5) can be used with good results. Fourth, the opticallymodified material 28 is formed into an optical article or lens 10 of theinvention.

Referring to FIGS. 4 and 5, two methods of dispersing the nanoparticlesinto the host material are schematically illustrated. According to FIG.4, an outline of the process for dispersion through compounding 32 isdepicted. In compounding 32, the selected nanoparticles 36 are fed intoa compounder 40, such as a twin screw extruder or a Farrell continuousmixer, along with pellets of the selected host material 34. Aftercompounding 40, the optically modified material is pelletized 42 for usein an injection molding machine (not shown). As shown in FIGS. 4 and 5,a surface treatment 38 and 52, respectively, may be needed to make thenanoparticles 36 compatible with the host material 34. Skilled artisanswill appreciate that this treatment could be applied to thenanoparticles 36 directly or added as an additive to the compounder 40along with the nanoparticles 36 and the host material 34.

According to FIG. 5, in the solvent-based dispersion process 44, theselected host plastic material 46 and the selected nanoparticles 48 aredispersed in solvents 50, 54, respectively, prior to mixing 56 the twosolvent solutions. The selected nanoparticles 48 are preferably exposedto an intermediate solvent dispersion step 54 to insure that a gooddispersion is obtained and all agglomerates are broken up. After mixingthe two solvent solutions together in step 56, the solvents are removedin step 58 and the optically modified material is pelletized 60 for usein an injection molding machine (not shown).

Following both techniques for making the optically modified material,the end result is plastic pellets which contain fully dispersednanoparticles such as shown in FIG. 2b with the nanoparticles beingpresent in sufficient quantity to deliver the reduced dn/dT desired.

Injection molding, compression molding and casting are the threepreferred techniques for forming the optical article 10 (refer to FIG. 3step 28) of the invention.

In a preferred embodiment, the nanocomposite optical article ofmanufacture 10 is comprised of a polymeric host material selected fromthe group consisting of thermoplastic materials and thermoset materials.Thermoplastic materials used in optical articles include:polymethylmethacrylate, polycarbonate, polystyrene, polysulfone, cyclicolefins, and blends and copolymers of those listed. Thermoset materialsused in optical articles include: diallyl glycolcarbonate, epoxides, andthermoset polyesters.

Typically the reduced dn/dT article of manufacture 10 produced withinthe contemplation of the invention are simple lenses, an array oflenses, opthalmic lenses, window glazing, optical fibers, cover glassesfor digital imagers, microlenses on digital imagers, and other opticaldevices of the like.

Skilled artisans will appreciate that modification of the opticalproperties of the host material is achieved, in accordance with themethod of the invention, by reducing the dn/dT of the nanocompositematerial. In our preferred embodiment, this is achieved by dispersing ananoparticulate material filler having a dn/dT with a sign that isopposite that of the base plastic.

EXAMPLE 1

An exemplary example of the aforementioned procedure for reducing thedn/dT of an optical plastic follows.

Polymethylmethacrylate nanocomposite optical plastic comprises apolymethylmethacrylate host material having a temperature sensitiveoptical vector x₁ and a magnesium oxide nanoparticles having atemperature sensitive optical vector x₂ dispersed in thepolymethylmethacrylate host material. According to the requirements ofthe invention, x₁ is directionally opposed to x₂.

More particularly, a polymethylmethacrylate host material is opticallymodified with the addition of magnesium oxide nanoparticles.Polymethylmethacrylate has a dn/dT of approximately −110E-6/° C. asshown in Table 1. Magnesium oxide has a dn/dT of approximately +19E-6/°C. Magnesium oxide nanoparticles are available from Nano MaterialsResearch in the 10 nm size. Magnesium oxide is transparent in the region0.35-6.8 micron which includes the visible light region. The volume (%)of magnesium oxide nanoparticles required in the polymethylmethacrylatehost material to reduce the dn/dT by 50% can be calculated based onvolume using Equation 2, below.

v ₅₀=0.5(γ_(p)/γ_(p)−γ_(n))  Equation 2

Wherein, V₅₀ is the volume % of the nanoparticles needed to reduce thedn/dT of the nanocomposite by 50% compared to the host plastic, γ_(p) isthe dn/dT of the host plastic (See FIG. 1); γ_(n) is the dn/dT of thenanoparticle material.

For the combination of polymethylmethacrylate and magnesium oxide, thevolume (%) of nanoparticles needed to reduce the dn/dT of thenanocomposite by 50% compared to the dn/dT of the polymethylmethacrylateis approximately 42%.

Referring to FIG. 4, magnesium oxide nanoparticles were compounded intopolymethylmethacrylate. In this case, a compatibilizer additive,Solsperse 21000 from Avecia Ltd. at 10% by weight of the nanoparticleswas mixed in with the polymethylmethacrylate pellets to aid indispersing the magnesium oxide nanoparticles. Compounding was done in atwin screw extruder. Lenses were then molded from the pellets producedfrom compounding. The resulting dispersion of the nanoparticles in thelenses was quite good when examined under the scanning electronmicroscope.

EXAMPLE 2

Alternatively, the nanocomposite material above was prepared using asolvent based dispersion process as shown schematically in FIG. 5, withtoluene or xylene. The solvent based dispersion process has beensuccessful for wide variety of polymers (polymethylmethacrylate,polystyrene, polycarbonate and cyclic olefin) as well as a variety ofnanoparticles (titanium dioxide, magnesium oxide and zinc oxide). Thedispersion of the nanoparticles is accomplished in a mill to break upthe agglomerates. As a result, well-dispersed solutions have beenproduced.

Referring again to FIG. 5, solvent removal 58 can be accomplished atmoderate temperature with vacuum. The dried material is then run throughan extruder to form pellets. The pellets are then injection molded intooptical articles using the process in Example 1.

EXAMPLE 3

In another case, a polycarbonate host material is optically modifiedwith the addition of aluminum oxide nanoparticles. Polycarbonate has adn/dT of approximately −114E-6/° C. as shown in Table 1. Aluminum oxidehas a dn/dT of approximately +14E-6/° C. Aluminum oxide nanoparticlesare available from Kemco International Associates in the 37 nm size.Aluminum oxide is transparent in the region 0.19-5.0 micron whichincludes the visible light region. The volume (%) of aluminum oxidenanoparticles required in the polycarbonate host material to reduce thedn/dT by 50% can be calculated based on volume using Equation 2, below.

v ₅₀=0.5(γ_(p)/γ_(p)−γ_(n))  Equation 2

Wherein, v₅₀ is the volume % of the nanoparticles needed to reduce thedn/dT of the nanocomposite by 50% compared to the host plastic; γ_(p) isthe dn/dT of the host plastic (See FIG. 1), γ_(n) is the dn/dT of thenanoparticle material.

For the combination of polycarbonate and aluminum oxide, the volume (%)of nanoparticles needed to reduce the dn/dT of the nanocomposite by 50%compared to the dn/dT of the polycarbonate is approximately 45%.

EXAMPLE 4

In another case, a polystyrene host material is optically modified withthe addition of aluminum oxide nanoparticles. Polystyrene has a dn/dT ofapproximately −127E-6/° C. as shown in Table 1. Aluminum oxide has adn/dT of approximately +14E-6/° C. Aluminum oxide nanoparticles areavailable from Kemco International Associates in the 37 nm size.Aluminum oxide is transparent in the region 0.19-5.0 micron whichincludes the visible light region. The volume (%) of aluminum oxidenanoparticles required in the polycarbonate host material to reduce thedn/dT by 50% can be calculated based on volume using Equation 2, below.

 v ₅₀=0.5(γ_(p)/γ_(p)−γ_(n))  Equation 2

Wherein, v₅₀ is the volume % of the nanoparticles needed to reduce thedn/dT of the nanocomposite by 50% compared to the host plastic, γ_(p) isthe dn/dT of the host plastic (See FIG. 1); γ_(n) is the dn/dT of thenanoparticle material.

For the combination of polycarbonate and aluminum oxide, the volume (%)of nanoparticles needed to reduce the dn/dT of the nanocomposite by 50%compared to the dn/dT of the polycarbonate is approximately 45%.

EXAMPLE 5

In another case, a cyclic olefin homopolymer host material is opticallymodified with the addition of magnesium oxide nanoparticles. Cyclicolefin homopolymer has a dn/dT of approximately −110E-6/° C. as shown inTable 1. Magnesium oxide has a dn/dT of approximately +14E-6/° C.Magnesium oxide nanoparticles are available from Nano Materials Researchin the 10 nm size. Magnesium oxide is transparent in the region 0.35-6.8micron which includes the visible light region. The volume (%) ofmagnesium oxide nanoparticles required in the cyclic olefin homopolymerhost material to reduce the dn/dT by 50% can be calculated based onvolume using Equation 2, below.

v ₅₀=0.5(γ_(p)/γ_(p)−γ_(n))  Equation 2

Wherein, v₅₀ is the volume % of the nanoparticles needed to reduce thedn/dT of the nanocomposite by 50% compared to the host plastic; γ_(p) isthe dn/dT of the host plastic (See FIG. 1), γ_(n) is the dn/dT of thenanoparticle material.

For the combination of cyclic olefin homopolymer and magnesium oxide,the volume (%) of nanoparticles needed to reduce the dn/dT of thenanocomposite by 50% compared to the dn/dT of the cyclic olefinhomopolymer is approximately 43%.

The invention has therefore been described with reference to a preferredembodiment thereof. However, it will be appreciated that variations andmodifications can be effected by a person of ordinary skill in the artwithout departing from the scope of the invention.

PARTS LIST

1 prior art lens

3 incident light

5 focal point of the lens

7 range of change of the focal length produced by a change intemperature of the plastic lens

10 nanocomposite lens

12 reduced range of change of the focal length produced by a change intemperature of the nanocomposite lens

14 dispersed nanoparticles

15 representation of nanoparticles dispersed into the host plasticmaterial

16 plastic host material

20 schematic of method for making reduced dn/dT article

22 step of selecting host material

24 step of selecting the nanoparticulate material

26 step of dispersion

28 step of forming an optical article

32 compounding process

34 step of selecting host material

36 step of selecting nanoparticulate material

38 step of surface treating nanoparticles

40 step of compounding nanoparticles

42 step of forming the nanocomposite into a useable form such aspelletizing

44 solvent-based dispersion process

46 step of selecting host material

48 step of selecting nanoparticles

50 step of dissolving host material in solvent

52 step of surface treating nanoparticles

54 step of dispersing nanoparticles in solvent

56 step of mixing together products of steps 50 and 54

58 step of removing the solvent

60 step of forming a useable article

What is claimed is:
 1. Polystyrene nanocomposite optical plastic article, comprises: a polystyrene host material having a temperature sensitive optical vector x₁ and magnesium oxide nanoparticles dispersed in said polystyrene host material having a temperature sensitive optical vector x₂, wherein said temperature sensitive optical vector x₁ is directionally opposed to said temperature sensitive optical vector x₂, and wherein said magnesium oxide nanoparticles have a volumetric loading in said host material defined by the relation v ₅₀=0.5(γ_(pi)γ_(p)−γ_(n)) wherein:v₅₀ is the volume % of the magnesium oxide nanoparticles required to reduce dn/dT of the nanocomposite by 50% compared to the host material; γ_(p) is the dn/dT of the host plastic, and, γ_(n) is the dn/dT of the magnesium oxide nanoparticle material.
 2. The polystyrene nanocomposite optical plastic article recited in claim 1 wherein each of said temperature sensitive optical vectors x₁ and x₂ are defined by a change in refractive index (dn) of said polystyrene host material and said magnesium oxide nanoparticles, respectively, with respect to a change in temperature (dT).
 3. The polystyrene nanocomposite optical plastic article recited in claim 1 wherein said temperature sensitive optical vector x₁ has a negative value of about 114×10⁻⁶/degree C and said temperature sensitive optical vector x₂ has a positive value of greater than about 6×10⁻⁶/degree C and less than about 50×10⁻⁶/degree C.
 4. The polystyrene nanocomposite optical plastic article recited in claim 1, wherein said predetermined volume (%) of said magnesium oxide nanoparticles dispersed in said polystyrene host material is about 43%.
 5. A method of manufacturing a polystyrene nanocomposite optical plastic article, comprising the steps of: (a) providing a polystyrene host material having a temperature sensitive optical vector x₁ and magnesium oxide nanoparticles having a temperature sensitive optical vector x₂, wherein said temperature sensitive optical vector x₁ is directionally opposed to said temperature sensitive optical vector x₂; (b) dispersing said magnesium oxide nanoparticles into said polystyrene host material forming a polystyrene nanocomposite material; and, (c) forming said polystyrene nanocomposite material into said polystyrene nanocomposite optical plastic article. 